The present invention relates to an optical flow sensor for measuring the flow of a fluid. More particularly, the optical flow sensor is a thermal dilation type sensor where a probe is heated and the temperature to which the probe is heated varies according to the flow of fluid past the probe. The invention more particularly relates to a Michelson type optical fiber interferometer and its application in measuring the temperature of the probe.
In order to measure the flow of a fluid accurately, it is necessary to employ a flow sensor that measures mass flow. There are two approaches to achieve this measurement: a Cordilis based flow sensor and thermal dilation flow sensor. In the former type, electrical power is required to vibrate the Cordilis sensing element whereas conventional thermal dilation sensors require electric power at the sensor head to heat the probe. These sensors have a substantial amount of metal (e.g. thermocouples, electric wires, etc.) and thus are susceptible to electromagnetic pickup and radiation, and have a sparking potential. Such a sensor would be undesirable when measuring, for example, the flow of a fuel-air mixture into a combustion type engine, or when located in close proximity to circuitry whose operation could be harmed by spurious electromagnetic radiation. This is especially important in areas where space is at a premium, such as aircraft or an oceangoing vessel.
There are examples of non-standard flow sensors which are relevant to the problems discussed above. For example:
U.S. Pat. No. 4,918,492 to Ferdinand et al. describes an interferometer for the measurement of temperatures in, for example, turbo-machines. One arm of the interferometer terminates in a sensor sensitive to the physical phenomenon to be evaluated and comprised of a hollow and open cell for receiving a part of a fluid to be measured and a mirror for returning the measurement optical wave. Ferdinand et al. do not disclose a heating means which is requisite for constructing a dilation type flow sensor.
U.S. Pat. No. 3,683,692 to Lafitte describes an apparatus to compute and measure the flow of a gaseous fluid by measuring a quantity of heat necessary to raise the temperature of a fluid of a given quantity, comprising a sensing means disposed in the gaseous flow and a reference means disposed in a dead-end cavity in such a manner as to be insensitive to the flow of fluid in this cavity. The sensing means and the reference means comprise a heating resistor to continually heat the fluid in order to raise its temperature and a detecting element sensitive to the temperature, the sensing means also including a heat compensating resistor whose electrical current supply is regulated by a lack of balance between the two detection elements for maintaining the elevation of temperature of the fluid flowing past the sensing means, and a means to continually measure the amount of current passing through the heat compensating resistance. The Lafitte apparatus uses electrical heating means which may be dangerous around fuel-air mixtures.
U.S. Pat. No. 4,755,668 to Davis describes a fiber optic interferometric thermometer with serially positioned fiber optic sensors comprising a single optical fiber and a means for enabling a temperature to vary the phase of light in several well-specified regions of the optical fiber. The sensing system consists of a Fabry-Perot type interferometer connected at one arm to the end of the optical fiber sensor. The optical fiber sensor is separated from the remainder of the optical fiber by a half-silvered mirror. The other end of the sensor region is fully mirrored. Thus, light is divided by the half-silvered mirror, so that one part of the light incident on the sensor is reflected back toward the coupler by the half-silvered mirror and constitutes the reference beam. The other part is transmitted into the optical fiber sensor portion and constitutes the sensor beam. The sensor beam experiences an added phase shift compared to the reference beam due to an added path length and the effect of the parameter being measured. The sensor component of the beam is then reflected by the full mirror at the end and passes once more through the sensor region experiencing an additional phase shift and is transmitted back through the half-silvered mirror and is interferometrically combined at the half-silvered mirror with the reference beam initially reflected by the half-silvered mirror. The fiber optic sensor region varies the phase of the light reflected from the mirrored end of the sensor according to the temperature of the sensor.
However, none of these sensors are fully satisfactory as a flow sensor, or for use in close proximity to electronic circuitry, or volatile chemicals. The present invention is aimed at eliminating these problems while at the same time affording greater flow measurement sensitivity.